The importance of chain terminating aromatic polycarbonate has been known for many years. Non chain terminated aromatic polycarbonates are insufficiently heat stable since the free phenolic end groups provide a reactive site which is detrimental to resin stability. The absence of a chain terminating agent or condition during the preparation of aromatic polycarbonate also leads to a polymer with an overall molecular weight which frequently is so high that the polymer is too viscous to mold at reasonable temperatures.
Standard chain terminating agents employed in the preparation of aromatic polycarbonate are members of the phenol family such as phenol itself and tert.butyl phenol. Other chain terminating agents have been disclosed but these moieties seem to have diminished certain of the positive characteristics of the aromatic polycarbonate. For example, German Offenlegungsschrift No. 27 16 304, laid open Oct. 19, 1978, discloses the use of a relatively long chain carboxylic acid or carboxylic acid halide (C.sub.9-26) in conjunction with the usual phenolic chain stopper. Although melt viscosity is lowered thereby permitting easier processing of the resin, a significant lowering of the heat resistance of the polymer as measured by Vicat temperature as well as embrittlement of the resin upon aging at elevated temperature also occur. Improved melt viscosity (fluidity) was also obtained in Japanese Kokai No. 76 34,992 laid open Mar. 25, 1976, when capryloyl chloride was employed as a chain terminating agent for an aromatic polycarbonate. However the fluidity comparison was to a non-chain terminated aromatic polycarbonate prepared under the same conditions. Reaction of the phenolic groups of non-chain terminated final polymerized polycarbonate has been undertaken in U.S. Pat. No. 3,475,373. In this patent, final polymerized polycarbonate was disclosed as being treated with an acyl halide followed by a hydroxy compound to combine with the excess halide. Various alkyl and aryl acyl halides as well as chloroformates were named as "typical acyl halides". Isopropyl chloroformate and isobutyryl chloride were included in the lists as "typical halides". Reaction of the end groups of the final polymer was undertaken since it was believed that the presence of chain terminator during the preparation of the polymer did not modify all of the characteristic hydroxyl or phenolic groups of the polymer. The high temperature oxidative stability of the polycarbonate polymer prepared according to the invention of the patent was compared with the non-chain terminated polymer.
In contrast to these earlier attempts to chain terminate an aromatic polycarbonate, the use of the branched alkyl acyl agents of this invention brings about a chain terminated aromatic polycarbonate with a higher heat resistance and better impact resistance after aging at an elevated temperature than the use of the straight alkyl acyl agents. The heat resistance and impact resistance after aging at an elevated temperature of aromatic polycarbonate chain terminated with the more highly branched agents of this invention is similar to or better than the aromatic polycarbonate chain terminated with a standard agent, phenol.